The present invention relates to oilfield operations and more particularly to a method for determining the quality of a cementing job. The term "cementing job" refers to the placing of a cement between a steel casing lowered into a borehole and the earth formation to isolate one formation from another formation. The casing also serves to prevent the formation from caving or falling into the bottom of the borehole. This is particularly done in the case where it is desirable to isolate the various formations to prevent undesired fluid communication between the formations.
Undesired fluid communication between formations is particularly a problem where fresh water formations are present. Obviously, any communication between a formation, such as a salt water formation and a fresh water formation, would contaminate the fresh water supply. Thus, it is important that the cement between the casing and the formation provide a fluid-tight seal and prevent any fluid communication between various formations.
Various methods have been developed to determine the quality and effectiveness of the cementing job. The most commonly used method is an acoustical evaluation process. In this process acoustical pulses are transmitted through the casing into the formation and the reflected signal is detected and recorded. The amplitude of the returning signal is analyzed to detect the presence of voids in the cement. A low amplitude signal normally indicates the presence of a void.
This acoustical evaluation fails to detect minor channels in the cement that would permit fluid flow between formations. The acoustical evaluation also fails to detect the presence of a mud sheath between the cement and the formation. The term 'mud sheath' is used to refer to the sheath or cake of solids contained in the drilling mud that is deposited along the borehole wall during drilling operations to prevent fluid inflow into the well during drilling. As is well known, drilling muds used in drilling operations contain suspended solids that may be deposited on the borehole wall to form an effective mudcake or seal. During cementing operations it is necessary to remove this mudcake to provide a good fluid seal between the cement and the borehole wall. Failure to remove the mudcake will leave a mud sheath which can provide a path for fluid communication between the formations and defeat the purpose of the cementing job. Conventional acoustic evaluation services do not detect the presence of a mud sheath since the mud sheath appears similar to conventional cement in the acoustic signal and does not appear as a void or other anomaly.
U.S. Pat. No. 4,035,640 discloses an alternate method to the acoustic method for detecting fluid channeling through the cement bond. In this patent there is disclosed a well tool having a fast pulse neutron source for irradiating the formations behind the well casing. The irradiation activates the oxygen nuclei which would be present in the water flowing in a channel in the cement bond. Gamma detectors are then used to detect the unstable isotope produced when the oxygen nuclei captures a slow neutron. While this system detects the presence o: channels in the cement bond in which fluid is flowing, it will not detect a static mud sheath in which there is no fluid flow. The mud sheath can be easily washed out over time by the formation fluid which would result in unwanted fluid flow between the formations.